1. Field of the Invention
The invention relates to a high density ink jet printhead and, more particularly, to a sidewall actuator for a high density ink jet printhead channel which imparts ink ejecting pressure pulses to the channel.
2. Description of Related Art
Printers provide a means of outputting a permanent record in human readable form. Typically, a printing technique may be categorized as either impact printing or non-impact printing. ribbon placed near the surface of the paper. Impact printing techniques may be further characterized as either formed-character printing or matrix printing. In formed-character printing, the element which strikes the ribbon to produce the image consists of a raised mirror image of the desired character. In matrix printing, the character is formed as a series of closely spaced dots which are produced by striking a provided wire or wires against the ribbon. Here, characters are formed as a series of closely spaced dots produced by striking the provided wire or wires against the ribbon. By selectively striking the provided wires, any character representable by a matrix of dots can be produced.
Non-impact printing is often preferred over impact printing in view of its tendency to provide higher printing speeds as well as its better suitability for printing graphics and halftone images. Non-impact printing techniques include matrix, electrostatic and electrophotographic type printing techniques. In matrix type printing , wires are selectively heated by electrical pulses and the heat thereby generated causes a mark to appear on a sheet of paper, usually specially treated paper. In electrostatic type printing, an electric arc between the printing element and the conductive paper removes an opaque coating on the paper to expose a sublayer of a contrasting color. Finally, in electrophotographic printing, a photoconductive material is selectively charged utilizing a light source such as a laser. A powder toner is attracted to the charged regions and, when placed in contact with sheet of paper, transfers to the paper's surface. The toner is then subjected to heat which fuses it to the paper.
Another form of non-impact printing is generally classified as ink jet printing. Ink jet printing systems use the ejection of tiny droplets of ink to produce an image. The devices produce highly reproducible and controllable droplets, so that a stored image data. Most ink jet printing systems commercially available may be generally classified as either a "continuous jet" type ink jet printing system where droplets are continuously ejected from the printhead and either directed to or away from the paper depending on the desired image to be produced or as a "drop on demand" type ink jet printing system where droplets are ejected from the printhead in response to a specific command related to the image to be produced.
Continuous jet type ink jet printing systems are based upon the phenomena of uniform droplet formation from a stream of liquid issuing from an orifice. It had been previously observed that fluid ejected under pressure from an orifice about 50 to 80 microns in diameter tends to break up into uniform droplets upon the amplification of capillary waves induced onto the jet, for example, by an electromechanical device that causes pressure oscillations to propagate through the fluid. For example, in FIG. 1, a schematic illustration of a continuous jet type ink jet printer 200 may now be seen. Here, a pump 202 pumps ink from an ink supply 204 to a nozzle assembly 206. The nozzle assembly 206 includes a piezo crystal 208 which is continuously driven by an electrical voltage supplied by a crystal driver 210. The pump 202 forces ink supplied to the nozzle assembly 206 to be ejected through nozzle 212 in a continuous stream. The continuously oscillating piezo crystal 208 creates pressure disturbances that cause the continuous stream of ink to break-up into uniform droplets of ink and acquire an electrostatic charge due to the presence of an electrostatic field, often referred to as the charging field, generated by electrodes 214. Using high voltage deflection plates 216, the trajectory of selected ones of the electrostatically charged droplets can be controlled to hit a desired spot on a sheet of paper 218. The high voltage deflection plates 216 also deflect unselected ones of the electrostatically charged droplets away from the sheet of paper 218 and into a reservoir 220 for recycling purposes. Due to the small size of the droplets and the precise trajectory control, the quality of continuous jet type ink jet printing systems can approach that of formed-character impact printing systems. However, one drawback to continuous jet type ink jet printing systems is that fluid must be jetting even when little or no printing is required. This requirement degrades the ink and decreases reliability of the printing system.
Due to this drawback, there has been increased interest in the production of droplets by electromechanically induced pressure waves. In this type of system, a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric material which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed so as to produce a droplet that issues from an orifice. Since the voltage is applied only when a droplet is desired, these types of ink jet printing systems are referred to as drop-on-demand. For example, in FIG. 2, a drop on demand type ink jet printer is schematically illustrated. A nozzle assembly 306 draws ink from a reservoir (not shown). A driver 310 receives character data and actuates piezoelectric material 308 in response thereto. For example, if the received character data requires that a droplet of ink is to be ejected from the nozzle assembly 306, the driver 310 will apply a voltage to the piezoelectric material 308. The piezoelectric material will then deform in a manner that will force the nozzle assembly 306 to eject a droplet of ink from orifice 312. The ejected droplet will then strike a sheet of paper 318.
The use of piezoelectric materials in ink jet printers is well known. Most commonly, piezoelectric material is used in a piezoelectric transducer by which electric energy is converted into mechanical energy by applying an electric field across the material, thereby causing the piezoelectric material to deform. This ability to distort piezoelectric material has often been utilized in order to force the ejection of ink from the ink-carrying channels of ink jet printers. One such ink jet printer configuration which utilizes the distortion of a piezoelectric material to eject ink includes a tubular piezoelectric transducer which surrounds an ink-carrying channel. When the transducer is excited by the application of an electrical voltage pulse, the ink-carrying channel is compressed and a drop of ink is ejected from the channel. For example, an ink jet printer which utilizes circular transducers may be seen by reference to U.S. Pat. No. 3,857,049 to Zoltan. However, the relatively complicated arrangement of the piezoelectric transducer and the associated ink-carrying channel causes such devices to be relatively time-consuming and expensive to manufacture.
In order to reduce the per ink-carrying channel (or "jet") manufacturing cost of an ink jet printhead, in particular, those ink jet printheads having a piezoelectric actuator, it has long been desired to produce an ink jet printhead having a channel array in which the individual channels which comprise the array are arranged such that the spacing between adjacent channels is relatively small. For example, it would be very desirable to construct an ink jet printhead having a channel array where adjacent channels are spaced between approximately four and eight mils apart. Such a ink jet printhead is hereby defined as a "high density" ink jet printhead. In addition to a reduction in the per ink-carrying channel manufacturing cost, another advantage which would result from the manufacture of an ink jet printhead with a high channel density would be an increase in printer speed. However, the very close spacing between channels in the proposed high density ink jet printhead has long been a major problem in the manufacture of such printheads.
Recently, the use of shear mode piezoelectric transducers for ink jet printhead devices have become more common. For example, U.S. Pat. Nos. 4,584,590 and 4,825,227, both to Fischbeck et al., disclose shear mode piezoelectric transducers for a parallel channel array ink jet printhead. In both of the Fischbeck et al. patents, a series of open ended parallel ink pressure chambers are covered with a sheet of a piezoelectric material along their roofs. Electrodes are provided on opposite sides of the sheet of piezoelectric material such that positive electrodes are positioned above the vertical walls separating pressure chambers and negative electrodes are positioned over the chamber itself. When an electric field is provided across the electrodes, the piezoelectric material, which is polled in a direction normal to the electric field direction, distorts in a shear mode configuration to compress the ink pressure chamber. In these configurations, however, much of the piezoelectric material is inactive. Furthermore, the extent of deformation of the piezoelectric material is small.
An ink jet printhead having a parallel channel array and which utilizes piezoelectric materials to construct the sidewalls of the ink-carrying channels may be seen by reference to U.S. Pat. No. 4,536,097 to Nilsson. In Nilsson, an ink jet channel matrix is formed by a series of strips of a piezoelectric material disposed in spaced parallel relationships and covered on opposite sides by first and second plates. One plate is constructed of a conductive material and forms a shared electrode for all of the strips of piezoelectric material. On the other side of the strips, electrical contacts are used to electrically connect channel defining pairs of the strips of piezoelectric material. When a voltage is applied to the two strips of piezoelectric material which define a channel, the strips become narrower and higher such that the enclosed cross-sectional area of the channel is enlarged and ink is drawn into the channel. When the voltage is removed, the strips return to their original shape, thereby reducing channel volume and ejecting ink therefrom.
An ink jet printhead having a parallel ink-carrying channel array and which utilizes piezoelectric material to form a shear mode actuator for the vertical walls of the channel has also been disclosed. For example, U.S. Pat. Nos. 4,879,568 to Bartky et al. and 4,887,100 to Michaelis et al. each disclose an ink jet printhead array in which a piezoelectric material is used as the vertical wall along the entire length of each channel in forming the array. In these configurations, the vertical channel walls are constructed of two oppositely polled pieces of piezoelectric material mounted next to each other and sandwiched between top and bottom walls to form the ink channels. Once the ink channels are formed, electrodes are deposited along the entire height of the vertical channel wall. When an electric field normal to the polling direction of the pieces of piezoelectric material is generated between the electrodes, the vertical channel wall distorts to compress the ink jet channel in a shear mode fashion.